It is known that a synchronous transmission belt, e.g. a timing belt of a piston engine, operates properly only when the belt is at a satisfactory tension, and to impart this tension to the belt, proposals have already been made for devices known as "fixed tensioners", i.e. tensioners which, once intalled, are permanently locked in an operative condition. Such devices which impart significant tension to the belt but which are not capable of absorbing fluctuations in tension give rise to significant fatigue in the belt, thereby reducing its lifetime and that of the members which it drives. Such "fixed tensioners" have thus already been replaced by devices that are said to be "automatic" or "dynamic" and that compensate variations in belt tension in operation not only when starting and stopping the engine, but also whenever a driven member is put into operation. Depending on the mode of operation of an actuator that acts on a tensioner wheel which is pressed against the belt, these known devices may be classified into "rotary" type devices such as described in FR-A-2 633 689, or "rotating thrust" type devices as described in U.S. Pat. No. 4,883,446, or finally "linear axial thrust" type devices as described, for example, in EP-A1-0 330 362 or FR-A-2 645 931. Because of the advantages provided by systems in which the actuator is driven with sliding motion only (as compared with devices in which the motion comprises both sliding and rotation or comprises rotation only), it is such devices which, in general terms, the invention seeks to improve.
Such tensioners comprise a cylinder body in which a piston is slidably mounted, thereby delimiting a pressure chamber and an oil supply chamber, at least one channel passing through the piston to provide communication between the two chambers, and a valve for opening or closing said channel. With respect to certain operating features thereof, such tensioners may be characterized by the relationship between variations in the force generated as a function of the piston displacement (in other words by the stiffness of the device), it being understood that behavior of the transmission as a whole is optimized by command of the stiffness during reverse displacement of the piston.
Consequently, a general object of the invention is to provide a belt tensioner, in particular for a synchronous transmission belt such as a timing belt for a piston engine, which, while presenting the advantages of "linear axial thrust" type devices, has improved operating characteristics over known devices, in particular with respect to the progressiveness with which force is applied.
In this respect, an object of the invention is to provide such a tensioner device which enables the lifetime of the belt with which it is associated to be increased.
Another object of the invention is to provide such a tensioner device which makes it possible, when associated with a toothed timing belt of a vehicle engine, to prevent the teeth jumping when cold starting the engine, and also to ensure that the tension in the belt is stable under all operating conditions of said engine.
Another object of the invention is to provide such a device that can easily be adapted to engine assemblies that differ from one another in shape and/or other characteristics, such that the same basic device can be easily modified to equip a range of synchronous transmissions.
Finally, an object of the invention is to provide such a tensioner device having a structure that is particularly simple, and is thus low in cost.